Knowledge of the porosities of earth formations surrounding a borehole is important in the petroleum industry to assist in identifying potential oil-and-gas-bearing locations. Epithermal neutron porosity logging which is one of the ways of investigating earth formations makes use of the facts that hydrogen strongly affects neutron moderation and that the pore spaces of earth formations are usually filled with hydrogen-rich fluids, namely hydrocarbons or water. In one form of epithermal neutron porosity logging, the borehole and formation are irradiated with neutrons from an accelerator source and populations of epithermal neutrons are detected and counted at one or more locations away from the neutron source as is shown for example in U.S. Pat. No. 5,051,581 (Hertzog et al.). The counted neutrons are correlated with porosity either individually or as ratios of counted neutrons. This form of epithermal neutron logging can be referred to as "spatial neutron porosity logging". One type of spatial neutron porosity logging measures a quantity commonly referred to as "the slowing down length" of the epithermal neutrons that irradiate the formation.
In another form of epithermal neutron logging, the borehole and formation are also irradiated with bursts of neutrons from an accelerator source and the time rate of decay (also known as die away) of epithermal neutrons is determined. In this type of measurement, epithermal neutron counts are correlated as a function of time. Such a logging technique is shown for example in U.S. Pat. No. 3,487,211 (Youmans). This form of epithermal neutron logging can be referred to as "temporal neutron porosity logging" and is also commonly known as "slowing down time" logging.
The advantage of spatial neutron porosity logging is that it provides a greater depth of investigation into the earth formation, i.e., the distance extending away from the detector and into the formation at which information is obtained, and is thus less affected by the borehole environment. Therefore, because of this greater depth of investigation, spatial neutron porosity measurements are typically less affected than temporal neutron porosity measurements by the tool standoff, i.e., distance between the detector and borehole wall, and other borehole conditions such as the type of fluids in the borehole and the invaded zone of the formation near the borehole. However, a disadvantage is that spatial measurements are normally strongly affected by earth formation lithology and matrix density.
The advantage of temporal neutron porosity logging is that those measurements are relatively insensitive to formation lithology and matrix density. However, a disadvantage with the temporal measurement is that, it is inherently a shallow measurement. Therefore, temporal measurements are strongly affected by the borehole conditions such as tool standoff, the type of fluid in the borehole and the extent of the invaded zone.
There have been several attempts to address the above stated disadvantages of both the spatial and temporal logging. In one solution, as described in the aforementioned U.S. Pat. No. 5,051,581 (Hertzog et al.), spatial and temporal measurements are combined to measure porosity. In this method, populations of epithermal neutrons are detected at near and far detector locations from a neutron source in the borehole and count signals indicative of epithermal neutron populations at the near and far locations are generated. Then a count ratio of the near detector neutron count to the far detector neutron count is generated. From neutron count ratio of these populations, the slowing down length (spatial measurement) of the neutrons is measured. The slowing down length measurement can be transformed into formation porosity measurement provided the lithology and matrix density of the formation are known. Temporal logging measurements are also obtained by determining the rate of decay and the time distribution of epithermal neutrons at one detector location. These temporal measurements are then processed to derive independent values of formation porosity. The spatial and temporal measurements are then combined to obtain a formation porosity measurement, which is substantially corrected for tool standoff.
The simultaneous measurements of both the spatial and temporal distributions of the epithermal neutrons provide porosity measurements more powerful than either measurement alone provides. As previously stated, a spatial measurement of porosity is normally very sensitive to formation lithology and matrix density and relatively insensitive to tool standoff. The slowing down time derived porosity is very sensitive to tool standoff effects and relatively insensitive to formation lithology and matrix density. As previously stated, the combination of the spatial measurement and slowing down time (temporal) measurement results in porosity values that are corrected for tool standoff. Also, the two measurements have different depths of investigation, and making both of them improves the measurement of the overall porosity variations.
Another solution is to combine the temporal measurements with other measurements as described for example in U.S. Pat. No. 5,068,531 (Allen et al). This patent discloses a method that attempts to produce a porosity measurement that is corrected for detector standoff. In the practice of the invention, the die away of nuclear radiation is measured and a model of the die away radiation is produced using exponential terms varying as the sum of the borehole formation and thermal neutron background components. Exponentially weighted moments of both die away measurements and die away models are determined and equated. The equated moments are solved for the ratio of the borehole to formation amplitude of the components. The formation die away constant is determined from at least the formation and thermal neutron background terms of the weighted measurement and model moments. The determined borehole to formation amplitude ratio is used to correct the determined formation die away constant for the effects of detector standoff from the borehole wall. A porosity log of the formation is produced which is hopefully corrected for detector standoff from the borehole wall as a function of the standoff corrected formation die away time constant calibrated in borehole models of known porosities.
Still another solution is to use signal analysis and decomposition techniques with temporal measurements as described, for example, in U.S. Pat. No. 4,600,838 (Steinman et al ) In this patent the detected indications of thermal neutron concentrations are processed by determining the zeroth order moment of the indications during the sequence of discrete time gates and determining the first order moment of the indications during the sequence of discrete time gates in order to obtain the desired neutron decay characteristic of the earth formation separate from that of the borehole.
Still other neutron logging techniques are described, for example in U.S. Pat. Nos. 4,973,839 (Nelligan) and 4,927,082 (Loomis et al.). Nelligan describes a method of determining the water saturation in the formation fluid and can identify the existence of hydrocarbons in the formation without determining the formation porosity. Loomis derives porosity from processing detector count rate measurements of the epithermal neutron die away curve in accordance with a multi-parameter fit to obtain the epithermal neutron decay time of the formation. Other more complex signal decomposition processing techniques are described for example in PCT application WO 89/11108, (Allen and Mills), published Nov. 16, 1989.
Although, these solutions may be effective, there are problems with each solution. Additional process steps are required in taking multiple porosity measurements and performing additional correction processing. Often additional information extraneous to the tool is needed to be combined with the spatial and temporal measurements and may not be available. Also, to obtain this additional information may require use of another logging tool in the formation at additional expense and time. Also, a problem with the approach using signal decomposition of the die away of neutrons is that this approach is affected by the shallow depth of investigation.
Therefore, there remains a need for a simple and accurate method and apparatus of performing epithermal neutron porosity logging that is less affected by the borehole environment and also has a reduced sensitivity to the effects of formation lithology and matrix density effects.